1. Field of the Invention
The present invention relates to an optical waveguide electro-optic device and a process of manufacturing the optical waveguide electro-optic device, and especially relates to an improved optical waveguide electro-optic device which is suitable for miniaturization and a manufacturing process thereof.
2. Description of the Related Art
Traditionally, as an electro-optic device having an electro-optical effect, an optical waveguide electro-optic device and a manufacturing process thereof are known. The optical waveguide electro-optic device is used for such as an intensity modulator used in optical communication, a Q switch element used for obtaining pulse behavior of laser oscillation, and an optical deflector for controlling a traveling direction of light.
Here, a phenomenon in which a refractive index of a material changes by applying an electric field to the material is called an electro-optical effect. When a refractive-index variation Δn due to the electro-optical effect is a Pockels effect, which is a linear electro-optical effect, the following formula is given.Δn∝rij×V/d  (1)
Here, rij represents an electro-optic constant (a Pockels constant), V represents an applied voltage, and d represents an interval of electrodes applied with the voltage.
It is desirable that such an electro-optic device respond at a low voltage and in a wide frequency domain. Large refractive-index variation at a low voltage is necessary, to obtain response characteristics at a low voltage and in a wide frequency domain. It is known from the formula (1) that this requirement can be met by decreasing the electrode interval d to increase the electric field strength (V/d). Accompanying a decrease in the electrode interval, the electro-optic device is usually formed with an optical waveguide structure.
As a conventional typical optical waveguide electro-optic device, for example, one formed by impurity diffusion is known, in which titanium diffuses locally in a crystal of lithium niobate to raise the refractive index of only the diffused region, to form the optical waveguide structure. In this optical waveguide structure, an electrode is formed near an optical waveguide area formed by diffusion of the impurity to a surface of the crystal.
In addition, as a general optical waveguide electro-optic device, a structure is known in which an adhesion layer, a lower electrode layer, a lower clad layer, a core layer, an upper clad layer, and an upper electrode layer are laminated sequentially on a support substrate.
In this kind of optical waveguide electro-optic device, an optical waveguide structure is produced that joins an electro-optical material or a nonlinear material to the support substrate formed of a material different from the above-mentioned materials directly or adheres them through an adhesive, and then the electro-optical material or the nonlinear material is thinned by grinding, and the upper electrode layer and the lower electrode layer are formed by a film-forming method such as a sputtering method or a vacuum deposition method.
Like this, in this kind of optical waveguide electro-optic device, the support substrate is necessary; therefore, a device which extracts the lower electrode layer arranged between the core layer and the support substrate to the exterior is necessary.
JP 4335542 B proposes that, in a conventional optical waveguide electro-optic device, as a device which is configured to extract a lower electrode layer arranged between a core layer and a support substrate to the exterior, for example, an area of a lamination plane of the support substrate is formed to be bigger than that of the core layer, and an extraction electrode which extracts the lower electrode layer to the exterior is provided on the support substrate.
JP 4335542 B discloses an optical deflection element by which a larger deflection angle than previously achievable can be obtained and a process of manufacturing the optical deflection element, in the optical waveguide electro-optic device. However, in the optical waveguide electro-optic device according to JP 4335542 B, when producing the optical deflection element having the larger deflection angle, the electro-optic device increases in size since the area of the support substrate has to be formed larger than that of the core layer.